Current approaches for transmitting data by inductive coupling using active load modulation can include resynchronizing a frequency of a periodic signal to that of an external magnetic field after each application of a burst (periodic signal burst, signal burst, etc.) to the antenna circuit, instead of resynchronizing this signal before each new data frame. Such active load modulation approaches can, therefore, include alternating periods of emitting bursts and phases (periods) of resynchronization to the external magnetic field.
However, in such approaches, after applying a burst of the periodic signal to the antenna circuit, the antenna signal can have a ringing that is superimposed on the “useful” antenna signal induced by the external magnetic field, which can adversely affect the resynchronization process by interfering with the external field, which is used as a resynchronization signal for the resynchronization process. That is, if the antenna signal includes oscillatory residues (transient oscillations) from a burst that has just been applied to the antenna circuit, those oscillatory residues can be superimposed on the useful antenna signal induced by the external field and can adversely affect the resynchronization process to the external field.
For example, FIG. 1 illustrates a shape of a burst B1 of a periodic signal Slm applied to an antenna circuit ACT, and the resulting antenna signal Vam for such current approaches. The antenna circuit ACT is a resonant circuit tuned to a frequency of the periodic signal Slm, and includes, for example, an antenna coil AC, a series capacitor Ca and a parallel capacitor Cb. The burst B1, or “incident burst”, produces, in the antenna circuit, a burst B1′ of antenna signal Vam, or a “resulting burst”, which then generates a burst of magnetic field of the same (substantially the same) shape as the burst B1.
The incident burst B1 in FIG. 1 is delimited in its duration and amplitude by an envelope signal E1 that is square in shape, and of duration T1, having a rising edge and a falling edge. The rising edge extends between a low inflection point it and a high inflection point i2. The falling edge extends between a high inflection point i3 and a low inflection point i4. The amplitude of the periodic signal Slm is zero (approximately zero) before the rising edge and after the falling edge, and is generally constant between the two edges. As shown in FIG. after applying the incident burst B1, the antenna signal Vam has a transient oscillation 1 of non-insignificant amplitude that may, in certain cases, have overshoots 2 of an amplitude greater than the maximum amplitude of the antenna signal Vam during the application of the incident burst B1. As a result, the resulting burst B1′ has a duration T1′ that may be greater than the duration T1 of the incident burst B1. When the time T1′-T1 is equal to or greater than a time separating the emission of two bursts B1, the device that emits the bursts B1 cannot be resynchronized to an external field, at the risk of resynchronizing to the signal the device is generating.
To overcome this drawback, current approaches can include short-circuiting or de-tuning the antenna circuit ACT by means of a switch (e.g., to electrical ground), which short-circuiting or de-tuning can occur immediately after applying the incident burst B1. These active load modulation approaches can then include, after applying the burst B1 and before the resynchronization phase, a damping phase during which the antenna circuit is short-circuited or de-tuned by the switch, followed by a restoration phase during which the useful antenna signal induced by the external magnetic field is restored by the external magnetic field without being “polluted” by (being superimposed with) the transient oscillation generated by the incident burst B1.
In such approaches, however, the damping switch must be able to withstand antenna voltages that can reach 10 to 15 V. Some technologies that are commonly used to produce circuits for using inductive coupling to transmit data (e.g., near-field communication (NFC) devices that are integrated on semiconductor chips), such as deep-submicron technologies, may not allow for producing transistors (switches) that are capable of withstanding such voltages without being damaged.
Accordingly, approaches for active load modulation that are implemented without the use of a damping switch are desirable.
In other words, it may be desirable to implement approaches for transmitting data by emitting bursts of magnetic field, in which the self-oscillation phenomenon of an associated antenna circuit is controlled by a mechanism other than a damping switch.